During the mechanical respiration of a patient by means of an anesthesia apparatus or a respirator, the pressure is reduced at the end of expiration to the extent that the patient can breathe out. The pressure level is not lowered now completely to the prevailing ambient pressure, but there remains a residual pressure, the so-called positive end-expiratory pressure (PEEP), in the lung. This pressure is maintained at a constant level by means of the anesthesia apparatus or respirator during expiration until the next inspiration. Any possible leaks in the connection from the anesthesia apparatus or respirator to the patient are compensated by a pressure regulator by the pressure regulator adjusting a volume by means of supplying a rate of flow to the extent that the positive end-expiratory pressure can be maintained in the patient's lungs. Due to this slight overpressure in the lungs against the ambient pressure, it is ensured by a distension of the lungs that the opened areas in the lungs will not collapse again during expiration during the time until the next inspiration and the exchange area of the lungs will not be reduced hereby.
A suctioning measuring system is connected in many cases by means of a flexible tube in anesthesia systems directly at the connection piece to the patient, in which the expiratory and inspiratory breathing tubes are brought together, the so-called Y-piece, and gas is sent with a suction volume flow directly from the patient into the measuring system, and the physiological respiration parameters of the patient are analyzed in the measuring system. For example, the oxygen concentration and the carbon dioxide concentration (CCO2) are recorded over time. Especially the carbon dioxide concentration (CCO2) at the end of expiration by the patient, the so-called end-tidal concentration (etCO2), is of diagnostic and therapeutic significance here. This makes it possible to infer how sufficient the patient's respiration, i.e., the supply with oxygen, is. The physician makes decisions about adjusting the respiration, for example, the respiration rate, the minute volume, the pressure settings, as well as the selected, administered oxygen concentration.
The adjustment of the positive end-expiratory pressure by a pressure regulator in the respiratory system causes fresh gas to be delivered directly to the patient to the so-called Y-piece. This fresh gas mixes there with the air expired by the patient. This mixed air is suctioned off from the patient, from the Y-piece, to the measuring arrangement. The suctioning takes place typically with a suction line, typically by means of a very thin suction tube with an internal diameter in the range of 0.5 mm to 1.5 mm over a length of 1.5-3.0 m. A suction volume flow of typically about 0.2 L/minute delivers the air through the suction tube from the patient into the measuring arrangement. The gas thus reaches the measuring arrangement with a delay in the range of about 0.8 sec to 3.5 sec due to its path and the type of suctioning. If additional components with an additional volume, for example, a water trap, are arranged in the suction line on the path to the measuring arrangement, and if the volume in the measuring arrangement itself, as well as the necessary measuring time for determining the carbon dioxide concentration in the measuring arrangement are taken into account, the delay between the location of the test sample at the patient and a value of a carbon dioxide concentration, which value is determined by measurement and displayed, increases, on the whole, to a value in the range of about 3 sec to 10 sec.
Such a delay corresponds approximately to a number ranging from less than one breathing cycle to two breathing cycles for an adult, to 1 to 6 breathing cycles for an infant as well as to 3 to 10 breathing cycles for a newborn.
Depending on the type of pressure regulation and the situation prevailing at the patient, at the respiratory system, the respiration parameters selected and the leaks present in the system, the regulated adjustment of the positive end-expiratory pressure causes the carbon dioxide concentration (CCO2) at the measuring arrangement not to correspond to the concentration that is present in the pharyngeal space and the bronchial space of the patient during expiration. A respiratory system with a regulator for setting a positive end-expiratory pressure is known from the document U.S. Pat. No. 4,082,093.